Development of materials for use as electrodes in optoelectronic devices, such as field effect transistors (FETs), light emitting diodes, photovoltaic devices (PVDs) and solar cells is an area of huge research interest. Traditionally, widely used electrode materials have been indium-tin-oxide (ITO) as a transparent front electrode with a metal, such as aluminium, barium, calcium, gold and the like, as a back electrode. However, ITO suffers from cracking and loss in conductivity when deposited on flexible substrates and subjected to bending. Furthermore, the fast development of the optoelectronic display industry has dramatically pushed up the price of indium, the raw material for the production of ITO electrodes. Alternatives to inorganic electrode materials such as ITO are urgently needed.
The provision of flexible electrode materials to address problems of cracking and consequent loss in conductivity seen with ITO electrodes is of great importance. Conducting polymer thin films are seen as an attractive alternative. Rapidly growing interest in polymer electronics has arisen from the promise of attaining lightweight, flexible electronic components that can be manufactured at low cost.
In recent years, poly(3,4-ethylenedioxythiophene) (PEDOT) has emerged as an excellent candidate material for flexible polymer electronics. PEDOT is a conducting polymer which has good stability and optical transparency in its conducting state. PEDOT itself is insoluble, but synthesis in the presence of the water soluble electrolyte poly(styrene sulfonic acid) (PSS), allows a stable PEDOT-PSS suspension to be formed that shows good film forming properties. When subjected to special treatment, such as secondary doping with glycerol or ethylene glycol, PEDOT-PSS films can have a conductivity which reaches 160 S/cm. This conductivity is, however, still far from the conductivity (in the region of 4000 S/cm) seen for ITO. PEDOT-PSS films are described in Jonsson, S. K. M., et al., Synthetic Metals, 2003. 139(1): 1-10, J, Huang., et al., Advanced Functional Materials, 2005. 15(2): 290-296 and J, Ouyang., et al., Polymer, 2004. 45(25): 8443-8450.
In order to seek further improvement in conductivity, chemical synthesis of PEDOT conducting polymer films has been widely investigated. Vapour phase polymerized PEDOT (VPP-PEDOT) films are particularly attractive, providing higher conductivity and transmission than PEDOT-PSS films. VPP-PEDOT synthesis is described in Jinyeol, K., et al. Synthetic Metals, 2003. 139(2): 485-489, Winther-Jensen, B., et al., Macromolecules 2004. 37(16): 5930-5935 and Winther-Jensen, B. and West, K.2004. Macromolecules 37(12): 4538-4543 and in WO2005/103109.
The polymerization process which leads to the formation of PEDOT involves (1) the oxidation of a 3,4-ethylenedioxythiophene (EDOT) monomer when an electron is withdrawn from an EDOT heteroaromatic ring, (2) the combination of two oxidized monomers to form a dimer with release of a proton, and (3) further oxidation of dimers and formation of trimers, etc, until long PEDOT chains are formed. The ionization potential of EDOT monomers and PEDOT dimers, trimers and infinite long chains are 1.1, 0.46. 0.16 and −0.25V (vs Ag/Ag+), respectively. Consequently, as soon as oligomers are formed, polymerization accelerates rapidly.
Existing VPP-PEDOT synthetic routes comprise three key steps: oxidant deposition, monomer polymerization and residual oxidant removal. Firstly, an oxidant layer is deposited on a substrate, generally glass or plastic, by spin coating or by gravure or screen printing methods carried out with a solution of an oxidant and an amine or amide polymerization inhibitor in an organic solvent. Following drying by heating, the substrate bearing an oxidant layer is transferred into a reaction chamber. The substrate bearing an oxidant layer is exposed to vapourized EDOT monomer in the reaction chamber. Polymerization takes place as the EDOT monomer vapour contacts the oxidant layer on the substrate, thereby forming a PEDOT film on the substrate surface. After the polymer film has formed, the substrate bearing a PEDOT film is washed to remove residual oxidant and any remaining polymerization inhibitor. Generally this washing is carried out with ethanol or methanol.
There are, however, several disadvantages to the existing VPP-PEDOT synthesis route described above. First, the PEDOT film synthesized on contact of the vapourized EDOT monomer with the oxidant layer has weak adhesion to the surface of the substrate. Thus, the PEDOT film easily loses contact with the substrate during the washing step. As a result of this, wrinkles may occur in the VPP-PEDOT film or the whole VPP-PEDOT film may peel off from the substrate into the wash solution. Second, due to the weak adhesion described above, it is not possible to thoroughly wash the VPP-PEDOT film and it is difficult to ensure complete removal of oxidant. This can cause problems with film morphology and may cause other problems when the film is used as an electrode. For example, the residual oxidant can crystallize as the temperature increases and cause deformation of the VPP-PEDOT film. The oxidant is also chemically reactive and therefore may cause degradation of the conjugated polymers, oligomers, dendrimer or other molecules to be used as the active layer in plastic electronic devices. Third, existing VPP-PEDOT synthetic routes generally require a large amount of organic solvent in the washing (oxidant removal) step. This is neither cost effective, nor environmentally friendly.
In order to obtain a VPP-PEDOT film with a smooth surface by using existing synthetic routes, it is necessary to immerse the substrate bearing a PEDOT film in organic solvent for a long time and use a large amount of solvent to wash the surface. The substrate must be handled with great care. Any quick movement of the substrate in the washing solvent may induce the tearing of the VPP-PEDOT film, or peeling off of the entire film into the solvent. In general, film morphology is sacrificed to ensure that the PEDOT film is maintained intact on the substrate.